BU-210: How does the Fuel Cell Work?

Explore the development of the fuel cell and study the different systems.

A fuel cell is an electrochemical device that combines hydrogen fuel with oxygen to produce electricity, heat and water. The fuel cell is similar to a battery in that an electrochemical reaction occurs as long as fuel is available. Hydrogen is stored in a pressurized container and oxygen is taken from the air. Because of the absence of combustion, there are no harmful emissions, and the only by-product is pure water. So pure is the water emitted from the proton exchange membrane fuel cell (PEMFC) that visitors to Vancouver’s Ballard Power Systems were served hot tea made from this clean water.

Fundamentally, a fuel cell is electrolysis in reverse, using two electrodes separated by an electrolyte. The anode (negative electrode) receives hydrogen and the cathode (positive electrode) collects oxygen. A catalyst at the anode separates hydrogen into positively charged hydrogen ions and electrons. The oxygen is ionized and migrates across the electrolyte to the anodic compartment, where it combines with hydrogen. A single fuel cell produces 0.6–0.8V under load. To obtain higher voltages, several cells are connected in series. Figure 1 illustrates the concept of a fuel cell.

Figure 1:
Concept of a fuel cell.
The anode (negative electrode) receives the hydrogen and the cathode (positive electrode) collects the oxygen.Source: US Department of Energy, office of Energy Efficiency and Renewable Energy

Fuel cell technology is twice as efficient as combustion in turning carbon fuel to energy. Hydrogen, the simplest chemical element (one proton and one electron), is plentiful and exceptionally clean as a fuel. Hydrogen makes up 90 percent of the universe and is the third most abundant element on the earth’s surface. Such a wealth of fuel would provide an almost unlimited pool of clean energy at relatively low cost. But there is a hitch.

With most fuels, hydrogen is bonded to other substances and “unleashing” the gas takes energy. In terms of net calorific value (NCV), hydrogen is more costly to produce than gasoline. Some say that hydrogen is nearly energy neutral, meaning that it takes as much energy to produce as it delivers at the end destination. (See BU-1007: Net Calorific Value.)

Storage of hydrogen poses a further disadvantage. Pressurized hydrogen requires heavy steel tanks, and the NCV by volume is about 24 times lower than a liquid petroleum product. In liquid form, which is much denser, hydrogen needs extensive insulation for cold storage.

Hydrogen can also be produced with a reformer by means of extraction from an existing fuel, such as methanol, propane, butane or natural gas. Converting fossil fuel into pure hydrogen releases some leftover carbon, but this is 90 percent less harmful than what comes from the tailpipe of a car. Carrying a reformer would add weight to the vehicle and increase its cost; reformers are also sluggish. The net benefit of hydrogen conversion is in question because it does not solve the energy problem.

With the availability of hydrogen through extraction, the fuel cell core (stack) to convert hydrogen and oxygen to electricity is expensive and the stack has a limited life span. Burning fossil fuels in a combustion engine is the simplest and most effective means of harnessing energy, but this contributes to pollution.

Sir William Grove, a Welsh judge and gentleman scientist, developed the fuel cell concept in 1839, but the invention never took off. This was during the development of the internal combustion engine (ICE) that showed promising results. It was not until the 1960s that the fuel cell was put to practical use during the Gemini space program. NASA preferred this clean power source to nuclear or solar power. The alkaline fuel cell system that was chosen generated electricity and produced drinking water for the astronauts.

High material costs made the fuel cell prohibitive for commercial use. The fuel cell core (stack) is expensive and has a limited life span. Burning fossil fuel in a combustion engine is the simplest and most effective means to harness energy, but it pollutes.

High cost did not discourage the late Karl Kordesch, the co-inventor of the alkaline battery, from converting his car to an alkaline fuel cell in the early 1970s. He mounted the hydrogen tank on the roof and placed the fuel cell and backup batteries in the trunk. According to Kordesch, there was enough room for four people and a dog. He drove his car for many years in Ohio, USA, but the only problem, Kordesch told me in person, was that the car did not pass inspections because it had not tail pipe.

Here are the most common fuel cell concepts.

Proton Exchange Membrane Fuel Cell(PEMFC)

The proton exchange membrane, also known as PEM, uses a polymer electrolyte. PEM is one of the furthest developed and most commonly used fuel cell systems; it powers cars, serves as a portable power source and provides backup power in lieu of stationary batteries in offices. The PEM system allows compact design and achieves a high energy-to-weight ratio. Another advantage is a relatively quick start-up when applying hydrogen. The stack runs at a moderate temperature of 80°C (176°F) and is 50 percent efficient. (The ICE is 25–30 percent efficient.)

On the negative, the PEM fuel cell has high manufacturing costs and a complex water management system. The stack contains hydrogen, oxygen and water, and if dry, water must be added to get the system started; too much water causes flooding. The stack requires chemical grade hydrogen; lower fuel grades can cause decomposition and clogging of the membrane. Testing and repairing a stack is difficult, given that a 150V stack requires 250 cells.

Freezing water can damage the stack and heating elements may be added to prevent ice formation. Start-up is slow when cold and the performance is poor at first. Excessive heat can also cause damage. Controlling temperatures and supplying oxygen requires compressors, pumps and other accessories that consume about 30 percent of the energy generated.

Operating a PEM fuel cell in a vehicle, the PEMFC stack has an estimated service life of 2,000–4,000 hours. Wetting and drying caused by short distance driving contributes to membrane stress. Running continuously, the stationary stack is good for about 40,000 hours. The stack does not die suddenly but fades similar to a battery. Stack replacement is a major expense.

Alkaline Fuel Cell (AFC)

The alkaline fuel cell has become the preferred technology for aerospace, including the space shuttle. Manufacturing and operating costs are low, especially for the stack. While the separator for the PEM costs between $800 and $1,100 per square meter, the same material for the alkaline system is almost negligible. (The separator for a lead acid battery costs $5 per square meter.) Water management is simple and does not need compressors and other peripherals; efficiency is in the 60 percent range. A negative is that the AFC is larger in physical size than the PEM and needs pure oxygen and hydrogen as fuels. The amount of carbon dioxide present in a polluted city can poison the stack and this limits the AFC to specialized applications.

Solid Oxide Fuel Cell (SOFC)

Electric utilities use three types of fuel cells, which are molten carbonate, phosphoric acid and solid oxide fuel cells. Among these choices, the solid oxide (SOFC) is the least developed, but it has received renewed attention because of breakthroughs in cell material and stack design. Rather than operating at the very high operating temperature of 800–1,000°C (1,472–1,832°F), a new generation of ceramic material has brought the core down to a more manageable 500–600°C (932–1,112°F). This allows the use of conventional stainless steel rather than expensive ceramics for auxiliary parts.

High temperature allows direct extraction of hydrogen from natural gas through a catalytic reforming process. Carbon monoxide, a contaminant for the PEM, is a fuel for the SOFC. Being able to accept carbon-based fuels without a designated reformer and delivering high efficiency poses significant advantages for this type of fuel cell. Cogeneration by running steam generators from the heat by-product raises the SOFC to 60 percent efficiency, one of the highest among fuel cells. As a negative, high stack temperature requires exotic materials for the core that adds to manufacturing costs and reduces longevity.

Direct Methanol Fuel Cell (DMFC)

Portable fuel cells have gained attention and the most promising development is the direct methanolfuel cell. This small unit is inexpensive to manufacture, convenient to use and does not require pressurized hydrogen gas. The DMFC has good electrochemical performance and refilling is done by squirting in liquid or replacing the cartridge. This enables continued operation without downtime.

Manufactures admit that a direct battery replacement by the fuel cell is years away. To bridge the gap, the micro fuel cell serves as a charger to provide continuous operation for the onboard battery. Furthermore, methanol is toxic and flammable, and there are limitations to how much fuel passengers can carry on an aircraft. In 2008 the Department of Transportation issued a ruling to permit passengers and crew to carry an approved fuel cell with an installed methanol cartridge and up to two additional spare cartridges of 200 ml (6.76 fl oz). This provision does not yet extend to bottled hydrogen.

Improvements are being made, and Toshiba unveiled prototype fuel cells for laptops and other applications generating 20 to 100 watts. The units are compact and the specific energy is comparable with that of a NiCd battery. Meanwhile, Panasonic claims to have doubled the power output with a similar size, specifying a calendar life of 5,000 hours if the fuel cell is used intermittently for 8 hours per day. The low longevity of these fuel cells has been an issue to be reckoned with.

Attempts are being made with small fuel cells running on stored hydrogen. Increased efficiency and smaller size are the advantages of pure hydrogen over methanol. These miniature systems have no pumps and fans and are totally silent. A 21cc cartridge is said to provide the equivalent energy of about 10 AA alkaline batteries with a runtime between refueling of 20 hours. This lends itself to portable computing, wireless communications and flashlights for the bicycle lone rider.

Military and recreational users are also experimenting with the miniature fuel cell. Figure 4 illustrates a portable fuel cell made by SFC Smart Fuel Cell. The EFOY fuel cell comes in different capacities that ranges from 600 to 2,160 watt-hours per day.

Figure 4: Portable fuel cell for consumer market.
The fuel cell converts hydrogen and oxygen to electricity and clean water is the only by-product. Fuel cells can be used indoors as an electricity generator.Courtesy of SFC Smart Fuel Cell AG
(2010)

Table 5 describes the applications and summarizes the advantages and limitations of common fuel cells. The table also includes the Molten Carbonate (MCFC) and Phosphoric Acid (PAFC), classic fuel cell systems that have been around for a while and have unique advantages.

Table 5: Advantages and limitations of various fuel cell systems.
Fuel cell developments have been gradual; the specific power is low and a direct battery replacement may never be feasible.

Developments

Limitations involve slow start-up times, low power output, sluggish response on power demand, poor loading capabilities, narrow power bandwidth, short service life and high cost. Similar to batteries, the performance of all fuel cells degrades with age, and the stack gradually loses efficiency. Such performance losses are much less apparent with the ICE.

Fuel cells below 1kW are normally non-pressurized and only use a fan to aid in oxygen supply; fuel cells above 1kW are pressurized and include a compressor that lowers efficiency and the system can get rather noisy. The relatively high internal resistance of fuel cells poses a further challenge. Each cell of a stack produces about 1 volt in open circuit; a heavy load causes a notable voltage drop. Similar to the battery, the power bandwidth decreases with age. Individual cells in the stack are also known to cause failures and contaminants are large contributors. Figure 6 illustrates the voltage and power bandwidth as a function of load.

Figure 6: Power band of a portable fuel cell
High internal resistance causes the cell voltage
to drop rapidly with load. The power band is limited to between 300 and 800mA.Courtesy of Cadex

Fuel cells operate best at a 30 percent load factor; higher loads reduce efficiency. This and poor throttle response place the fuel cell into a support mode or a charger to keep batteries charged. A stand-alone power source, as the developers had hoped, has not materialized.

Paradox of the fuel cell

The fuel cell enjoyed the height of popularity in the 1990s, when scientists and stock promoters envisioned a world running on a clean and inexhaustible resource — hydrogen. They predicted that cars would run on fuel cells, and that household electricity would also be generated by fuel cells. The stock prices skyrocketed but marginal performance, high manufacturing costs and limited service life moderated the hydrogen dream.

It was said that the fuel cell would transform the world as the microprocessor did in the 1970s. A clean and inexhaustible source of energy would become available that would solve the environmental concerns of burning fossil fuel. From 1999 through 2001, more than 2,000 organizations got actively involved in fuel cell development, and four of the largest public fuel cell companies in North America raised over a billion US dollars in public stock offerings. What went wrong?

Hydrogen is not a source of energy per se but a medium to transport and store energy similar to electricity that charges a battery. To envision “burning an endless supply of hydrogen,” the fuel must first be produced, because hydrogen cannot be pumped from the earth as is possible with oil. While fossil fuel lends itself well to producing hydrogen, taking this valuable fuel to unleash hydrogen makes little sense when it costs as much or more for extraction as burning it directly. The only benefit is reduced greenhouse gases.

Just as the attempt to fly airplanes on steam failed in the mid-1800s, it is conceivable that the fuel cell will never be the powerhouse scientists had hoped for. But there is renewed interest in the automotive field in Japan. Fuel cells are replacing battery banks and diesel generators in office buildings as they can be installed in tight storage places with minimal maintenance and without the need for exhaust. Fuel cells allow continuous and pollution-free operation of forklifts in warehouses, whereas 40M fuel cells generate clean electricity in remote locations. The ultimate dream is propelling vehicles with the clean fuel cell.

Fuel cells may one day taxi airplanes with electric wheel motors, lowering pollution and preserving fuel by up to 4 percent by not using the jet engines. Water produced from the fuel cell while charging the batteries could serve as on-board drinking water; generative braking could further assist in the charging. The ultimate dream is propelling vehicles with the clean fuel cell.

Last updated 2016-05-03

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Batteries in a Portable World

Comments

On June 4, 2011 at 7:52am

hugh spalding wrote:

Does adding water to sealed car batteries prolong battery life?

On October 14, 2011 at 8:17am

Al wrote:

As a fairly off-topic point, functional steam airplanes did exist in the 1930’s. The United States Post Office owned one. http://www.rexresearch.com/besler/beslerst.htm and a video here: http://www.archive.org/details/BeslerCo1932

On November 15, 2011 at 10:22pm

INBASEKAR wrote:

Very good but more latest fuelcells which can produce more power in terms of KW may be introduced

On November 15, 2011 at 10:23pm

INBASEKAR wrote:

micro grids may be introduced

On November 15, 2011 at 10:24pm

INBASEKAR wrote:

microbial fuel cells may be introduced

On November 15, 2011 at 10:25pm

INBASEKAR wrote:

microgrids interfacing fuel cells may be introduced

On January 18, 2013 at 3:29am

J wrote:

The price on energy, that will move us from one point to another, is rising and this energy source (fossil fuels)is also causing global warming and melting the ice.

The last couple of years I have seen an increase of renewable energy sources, wind and solar, but these will only give you heat or electricity to your house. There are systems for storing this energy in large tanks with liquid, you can later use it to heat your house and maybe with some kind of stirling engine produce electricity. The other option to store energy is to send it out the power grid and get payed or pay for your net power consumption.
One of the problems with solar energy is also that you dont need the energy when the sun shines (cold countries).
Would it be possible to create hydrogen if you had electric energy? You could making your own fuel at home.

On March 8, 2014 at 7:39pm

Michael Price wrote:

The text and first diagram are inconsistent. The text says the oxygen ion migrates across the electrolyte whereas the diagram shows the hydrogen ions migrating. Text is wrong?

The large amount of H2 needed for vehicle fuel cells could be made availiable by utilizing nuclear power plants extra capacity in off peak hours. Water, for a raw material is essentially free and, the process is non- polluting. If only we could get over our irrational fear of nuke plants.

On March 29, 2015 at 2:26pm

A. D. Kutluk wrote:

Instead of pressurazing hydrogen, it can well be produced from water (H2O) by way of hydrolize. ( A technic which was used in 1950s and forgetten for long but reinvogorated few yerars ago and now being used over 7000 cars as fuel supplement lowering level of consumption between 20-40% in Turkey)

On October 16, 2015 at 5:04am

prince onuoha wrote:

we will like to buy your Fuel cells that can be used indoors as an electricity generator

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